Danish term (edited): Brugeroprettelse / opret(te) bruger

This is what I would use IF the context is one where a user creates an account for themselves, as in on a website. Google, YouTube, Twitter, Wikipedia, Proz.com all use "Create an account". "User creation" is, in my experience, somewhat more technical, like when an administrator creates accounts for users on a server. This distinction seems to be less common in Danish than in English: Danish websites tend to use "opret bruger", "opret konto" and "opret profil" more or less interchangeably, where websites in English will generally just use "Create (an) account".